The invention relates to preventing or at least minimizing electrode contamination by trapping components of an electrode that leach from the electrode from contaminating another electrode in an assembly, and also to an assembly configured with a getter that traps the leached components.
Metals and alloys are often used to create electrodes in an electrochemical cell. Pt—Ru alloy have been used for anodes and Pt catalysts for cathodes for fuel cells. Similar catalysts have been used in direct methanol fuel cells (DMFCs) for increasing the methanol oxidation reaction rate. Another example is the use of a Ag—Au alloy for the cathode and a Ni catalyst for the anode of an alkaline fuel cell. The Ag—Au alloy improves oxygen reduction kinetics at the cathode while the Ni catalyst improves the hydrogen oxidation reaction rate at the anode. Polymer electrolyte fuel cells (PEFCs), including direct methanol fuel cells (DMFCs), and alkaline fuel cells (AFCs), including alkaline membrane fuel cells (AMFCs), have attracted great interest as an alternative power source for vehicles and portable electronic devices. Two major challenges facing PEFCs are the reduction of material costs and the need for improved performance. Regarding the former challenge, alloy and composite catalysts with low platinum content and non-platinum catalysts based on metals such as ruthenium, palladium, iron, manganese, cobalt, nickel, chromium, molybdenum, and vanadium have been investigated.
When a metal alloy catalyst is used in an anode for a membrane electrochemical assembly (MEA), it has been recognized that one or more components can leach from a first electrode of the assembly and migrate to a second electrode of the assembly. This results in electrode contamination. The component that migrates might not be considered a contaminant of the first electrode, but when it leaches from the first electrode and becomes a part of the second electrode, it contaminates the second electrode. For example, a metal oxide present in the anode of a fuel cell can leach from the anode and migrate to the cathode. The metal oxide might be part of the anode, but when it reaches and becomes a part of the cathode, it has become a contaminant. This contamination results in deteriorating catalytic activity, which leads to a decrease in oxygen reduction reaction (ORR) activity of such cathodes and a corresponding reduction in fuel cell performance. Leachable components include but are not limited to transition metals, non-metals, and organic components (e.g., PtCo, RuSe, Co-PPy) that are present in the first electrode (e.g. the cathode) that can leach from the first electrode and migrate to a second electrode (e.g. the anode) during operation of the electrochemical cell (e.g. a fuel cell). If the electrochemical cell is a fuel cell, this contamination hinders the ability of the now contaminated electrode to oxidize fuels (e.g., hydrogen, methanol, ethanol, and the like at the anode) or reduce oxygen (at the cathode).
A previous approach has involved the use of a sacrificial electrode called a getter to capture leachable components from a fuel cell anode (see: U.S. Pat. No. 7,575,824, incorporated by reference herein) by applying a potential of about 0.1 V between the anode and the sacrificial electrode. The applied potential enabled selective migration of the leachable component(s) to the getter. After trapping the leachable components, the getter was replaced with the fuel cell cathode. This method can effectively remove leachable components from a fuel cell electrode. However, the method required a rather labor intensive getter fabrication, and recycling of the getter has proven difficult and costly.
While contamination of fuel cell electrodes can adversely affect fuel cell performance, methods of preventing contamination are highly variable and may result in loss of electrode material. Therefore, there continues to be a need for a method of preventing contamination of fuel cell electrodes. What is also needed is a method of preventing contamination of fuel cell electrodes in situ in a membrane electrode assembly. In addition, what is needed is a method of improving fuel cell performance based on preventing contamination of an electrode of an assembly by leachable components from another electrode of the assembly.